old video game music legality?

Open source does not mean public domain. Open source just means that the code can be shared, modified, and redistributed, but usually there are restrictions, especially if you plan to sell it (other than the cost of the media used), and if you do distribute it, you must include all code, including any derivative that you created. The exact details on what you can and cannot do is part of the license for the open source code.

Public domain you can do anything you want with it, other than claim ownership of it. You can modify it (you can claim the modification as yours), you can copy it, you can sell it, all with no restriction.

You can download any music you like as long as you have a license or permission to do so. Usually this comes with the proviso that you pay a small fee in compensation. Usually it means that you cannot sell the music on, broadcast it or use it commercially.

Open source merely means that the game designers have allowed access to their software. They will have signed a license and paid fees to use any music on the games.

pft, please. No one is going to care if you violate the copy right of the old 8 bit video games. In fact if you want to play them on your pc, try dosbox com

and

dosgames com

often there some emulated games from those game systems you mentioned.